- Sites of sexual reproduction
- double fertilization
- Event leading to flowering are complex
- hormones
- biological clocks
- temperature
- photoperiod
- Flower production is coordinated with the environment
- necessary for pollination
- Dispersal of seeds is the avenue of plant distributions
Angiosperm reproduction: Flowers
- Flower is a modified stem for reproduction
- 4 types of modified leaves
- groped in whorls (outer → inner)
- Sepals: leaves at base which enclose flower
- calyx = all sepals together
- Petals: colored leaves to attract pollinators
- corolla = all petals together
- Two types of fertile organs to make spores
- androcieum = male whorl
- gynociem = female whorl
Flowers - Male structures - androcieum
- Whorl composed of stamens
- anther + stalk
- Anther: where pollen produced
- each anther has 4 chambers
- Filament: stalk that holds the anther
Flowers - Female structures - gynociem
- Whorl composed of carpels
- middle of flower
- 1 → many carpels
- Stigma: receives pollen
- sticky
- sticky
- Style: leads from stigma to ovary
- where pollen tube forms
- where pollen tube forms
- Ovary: contains ovules
- ovules becomes seeds
- 1 → many
Flowers - Female structures - gynociem
- Pistil also refers to 1 unit
- carpels can be separate or fused
- Complexity leads to diverse flowers
- also leads to diverse fruits
Variation in flower strucutre
- Complete flower: all four whorls
- Incomplete flower: missing at least one whorl
- Perfect flower: both stamens and pistils
- Imperfect flower: missing one or the other
- One plant w/ male & female flowers = moneocious
- If male & female flowers on separate plants = dioecious
- Regular flower = radial symmetry (rose)
- Irregular flower has bilateral symmetry (iris)
When to flower? Arabidpopsis has many known pathways
- Photoperiod: long days trigger flowering genes
- photoreceptor pigments
- Vernalization: prolonged cold induces flowering
- GA hormone: internal hormonal signals
- independent of environemnt
- Why so many?
Flowering in marijuana
- Flowering periods triggered by photoperiodism
- 6hrs or more of darkness
- vegetative growth synced w/ summer daylight
- Longer darkness cause hormonal change
- triggered by 2 photocrome pigments
- PR (660-700nm) & PFR (760-800nm)
- darkness switches PFR to PR
- tipping point of PFR:PR
Pollination
- Sex cells must travel from one plant to another
- sexual partners may be widely scattered
- Seed plants: pollen carried by wind, insects & birds
- inevitable pollen loss
- may also require nectar production
- Pollination = transfer of pollen from anther to stigma
- not fertilization!
- Some flowering plants can self-pollinate
- stamen & anther mature @ same time
- large genetic consequences
- no nectar necessary
Mechanisnms against self-pollination
- Many flowers have both male & female structures
- hermaphrodism
- Pollen and the ovary mature at different times
- Physical flower features that prevent self-pollination
- styles length relative to the stamens = heterostyly
- Biochemical blockers to reject closely related pollen
- e.g. enzymatic destruction of RNA
- chemical tests (proteins) on pollen tube
Evolution of flower color and smell
Alternation of generations: Where is the gametophyte?
- Male: microspore inside pollen grain
- microgametophyte = max 3 cells
- generative cell → mitosis → 2 sperm
- vegetative cell makes pollen tube
- Female: 7 cell embryo sac, inside ovule
- megaspore develops into megagametophyte
- mitosis 3x
- one cell has 2 nuclei
- one cell = egg
Angiosperm reproduction: double fertilization
Comparing seed plant reproduction
Fruits develop from ovaries
- Ovule develop into seeds
- Stigma, style, sepals & petals wither away
- 3 layers develop during growth
- exocarp = outer skin
- mesocarp = flesh
- endocarp = stone, pit or thin inner layer
Type of gynociem determines fruit type
Seeds as agents of dispersal
- Seeds capable of long-distance dispersal
- pros: promotes sexual reproduction
- cons: diverse landing sites
- 1 plant = 1,000s of seeds (possible)
- each a genetic experiment
- significant cost to plant
- Fruits/seeds by wind = light, wings or parachutes
- Fruits/seeds by water = buoyant & anti-rot
- Mechanisms to stick to animals
- Edible but not digested